Approaches to the engineering of hemoglobin based oxygen carriers

Molecular biology offers the opportunity to construct hemoglobin molecules as blood substitutes tailored to specific therapeutic applications. Oxygen affinity can be manipulated by amino acid substitutions in the heme pocket or on the protein surface. A response to the concentration of plasma-Cl- that mimics the effect of 2,3-DPG was also introduced in human Hb. Polymerization of tetrameric Hb prevents the vasoconstriction associated with infusion of Hb solutions. Polymers have been obtained through the formation of intermolecular S-S bonds between cysteine residues introduced on the Hb surface. In a mouse model, transfusion of polymeric hemoglobins reduced the volume of cerebral infarction. This effect was particularly evident with polymers having a high oxygen affinity (P50≤˜2.0 Torr) and no cooperativity (n = 1). These are the same functional characteristics of myoglobin. Polymers of myoglobin have been constructed and could be a viable alternative to the use of polymeric hemoglobins in some pathological conditions

PREPARATIVE AND ANALYTICAL APPLICATIONS OF IMMOBILIZED HEMOGLOBIN

New applications of haemoglobin immobilized on Sepharose 4B are proposed for the removal of cell-free, potentially toxic haemoglobin from either natural biological specimens or artificial haemoglobin-containing systems (e.g., blood substitutes, haemosomes) to be employed for biomedical purposes. In a model study, an affinity column of immobilized haemoglobin was used to remove free haemoglobin from blood serum in which controlled haemolysis had been induced. The affinity column retains all the free haemoglobin, does not retain the haemoglobin-haptoglobin complex(es) and leaves the composition of the serum samples unaltered. When immobilized met-haemoglobin is used, haem is transferred readily to albumin, with which it forms a complex. This observation on the one hand shows that immobilized oxyhaemoglobin should be preferred for preparative purposes, and on the other opens the way to the characterization of the haem transfer reaction to albumin by means of immobilized met-haemoglobin. This reaction is difficult to study in solution owing to the overlap of the met-haemoglobin and methaemalbumin spectra

Allosteric modulation by tertiary structure in mammalian hemoglobins. Introduction of the functional characteristics of bovine hemoglobin into human hemoglobin by five amino acid substitutions.

Bovine erythrocytes do not contain 2,3-diphosphoglycerate, the principal allosteric effector of human hemoglobin. Bovine hemoglobin has a lower oxygen affinity than human hemoglobin and is regulated by physiological concentrations of chloride (Fronticelli, C., Bucci, E., and Razynska, A. (1988) J. Mol. Biol. 202, 343-348). It has been proposed that the chloride regulation in bovine hemoglobin is introduced by particular amino acid residues located in the amino-terminal region of the A helix and in the E helix of the beta subunits (Fronticelli, C. (1990) Biophys. Chem. 37, 141-146). In accordance with this proposal we have constructed two mutant human hemoglobins, beta(V1M+H2deleted+T4I+P5A) and beta(V1M+H2deleted+T4I+P5A+A76K). These are the residues present at the proposed locations in bovine hemoglobin except for isoleucine at position 4. Oxygen binding studies demonstrate that these mutations have introduced into human hemoglobin the low oxygen affinity and chloride sensitivity of bovine hemoglobin and reveal the presence of a previously unrecognized allosteric mechanism of oxygen affinity regulation where all the interactions responsible for the lowered affinity and chloride binding appear to be confined to individual beta subunits

Introduction of a new respiratory mechanism into human hemoglobin

Previous studies on bovine hemoglobin (HbBv) have suggested amino acid substitutions, which might introduce into human hemoglobin (HbA) functional characteristics of HbBv, namely a low intrinsic oxygen affinity regulated by Cl(-). Accordingly, we have constructed and characterized a multiple mutant, PB5, [beta(V1M + H2 Delta + T4I + P5A + A76K)] replacing four amino acid residues of HbA with those present at structurally analogous positions in HbBv, plus an additional substitution, beta T4I, which does not occur in either HbBv or HbA. This 'pseudobovine' hemoglobin has oxygen binding properties very similar to those of HbBv: the P(50) of HbA, PB5 and HbBv in the absence of Cl(-) are 1.6, 4.6 and 4.8 torr, respectively, and in 100 mM Cl(-) are 3.7, 10.5 and 12 torr, respectively. Moreover, PB5 has 3-fold slower autoxidation rate compared to HbA and HbBv. These are desirable characteristics for a human hemoglobin to be considered for use as a clinical artificial oxygen carrier. Although the functional properties of PB5 and HbBv are similar, van't Hoff plots indicate that the two hemoglobins interact differently with water, suggesting that factors regulating the R to T equilibrium are not the same in the two proteins. A further indication that PB5 is not a functional mimic of HbBv derives from PB5(control), a human hemoglobin with the same substitutions as PB5, except the beta T4I replacement. PB5(control) has a high oxygen affinity (P(50)=2.3 torr) in the absence of Cl(-), but retains the Cl(-) effect of PB5. The Cl(-) regulation of oxygen affinity in PB5 involves lysine residues at beta 8 and beta 76. PB4, which has the same substitutions as PB5 except beta A76K, and PB6, which has all the substitutions of PB5 plus beta K8Q, both have a low intrinsic oxygen affinity, like HbBv and PB5, but exhibit a decreased sensitivity to Cl(-). Since HbBv has lysine residues at both beta 8 and beta 76, these results imply that Cl(-) regulation in HbBv likewise involves these two residues. The mechanism responsible for the low intrinsic oxygen affinity of HbBv remains unclear. It is suggested that residues peculiar to HbBv at the alpha(1)beta(1) interface may play a role